Olefin polymers comprising ethylene and at least one or more .alpha.-olefin and optionally one or more diolefin make up a large segment of polyolefin polymers and will be addressed for convenience as "ethylene copolymers" herein. Such polymers range from crystalline polyethylene copolymers to largely amorphous elastomers, with a new area of semi-crystalline "plastomers" in between. In particular, ethylene copolymer plastomers are becoming a well established class of industrial polymers having a variety of uses associated with their unique properties, such as elastomeric properties and their thermo-oxidative stability. Uses of the plastomers include general thermoplastic olefins, films, wire and cable coatings, polymer modification (by inclusion in blends with other polyolefins), injection molding, foams, footwear, sheeting, functionalized polymers (such as by free-radical graft addition of polar monomers) and components in adhesive and sealant compounds.
Commercially prepared ethylene copolymers have traditionally been made via Ziegler-Natta polymerization with catalyst systems largely based on vanadium or titanium. Newer metallocene catalyst compounds have received attention due to their ease of larger monomer incorporation and potential increases in polymerization activities. U.S. Pat. No. 324,800 describes metallocenes having substituted and unsubstituted cyclopentadienyl ligands which are suitable for producing high molecular weight olefin polymers, including linear, low density copolymers of ethynyl with minor amounts of .alpha.-olefin.
Noncoordinating anions useful as catalyst components with such metallocenes are known. The term "noncoordinating anion" is now accepted terminology in the field of olefin polymerization, both by coordination or insertion polymerization and carbocationic polymerization. The noncoordinating anions function as electronic stabilizing cocatalysts, or counterions, for cationic metallocenes which are active for olefin polymerization. The term "noncoordinating anion" as used here and in the references applies both to noncoordinating anions and weakly coordinating anions that are not so strongly coordinated to the cationic complex as to be labile to replacement by olefinically or acetylenically unsaturated monomers at the insertion site. U.S. Pat. No. 5,198,401 describes a preferred noncoordinating anion tetra(perflourophenyl) boron, [B(Pfp).sub.4 ].sup.31 or [B(C.sub.6 F.sub.5).sub.4 ].sup.-, wherein the perfluorinated phenyl ligands on the boron makes the counterion labile and stable to potential adverse reactions with the metal cation complexes. Other aryl radicals are said to be suitable; in addition to the phenyl radicals, napthyl and anthracenyl are listed. In a related European application EP 0 277 004, hafnocenes activated with anion providing catalyst precursor components are said to be preferred for high molecular weight products and for increased incorporation of olefins and diolefin comonomers with ethylene.
U.S. Pat. No. 5,296,433 teaches the utility of borane complexes comprising tris(pentafluorophenyl)borane and specific complexing compounds. These complexes are said to allow higher molecular weight polymers when used with metallocenes for olefin polymerization due to increased solubility of the complexes in monomer or monomer solutions. In particular, fluorenyl ligands on the metallocenes are said to be particularly useful for high molecular weight, rubbery polyolefins as observed from the degree of polymerization of poly-1-hexene with [(fluorenyl).sub.2 ZrMe].sup.+ [C.sub.18 H.sub.37 O.B(C.sub.6 F.sub.5).sub.3 ].sup.- in Table 1. WO 97/29845 describes the preparation of the organo-Lewis acid perfluorobiphenylborane, and its use to prepare and stabilize active, olefin polymerization catalysts. These cocatalysts are describe as being less coordinating than tris(perfluorophenyl)boron, B(C.sub.6 F.sub.5).sub.3, and thus capable of providing higher catalytic activities. Generic description of the suitable cocatalysts according to the application include those of the formula BR'R" where B is boron with R' being fluorinated biphenyl and R" representing at least one fluorinated phenyl, biphenyl or other polycyclic group, such as napthyl, anthryl or fluorenyl. These cyclic groups on the phenyl ligands are said to be suitable inn any of the ortho-, meta- or para-positions, but only the ortho-position is exemplified in the working examples.
The utility of metallocene-based ionic catalysts in high temperature olefin polymerization is described in U.S. Pat. No. 5,408,017, EP 0 612 768, WO 96/33227 and WO 97/22635. Each addresses suitable metallocene catalysts for high temperature processes for olefin copolymerization. High molecular weight ethylene/.alpha.-olefin copolymers is an objective of EP 0 612 768 and is addressed with catalyst systems based on bis(cyclopentadienyl/indenyl/fluorenyl) hafnocenes which are combined with an alkyl aluminum compound and an ionizing ionic compound providing a non-coordinating anion.
As described above, a recognized problem for high temperature polymerization, particularly where significant content of comonomer incorporation in ethylene copolymers is to be sought, is an observed decrease in molecular weight, or increase in melt index. Means of maintaining high molecular weights, or low M.I., in ethylene copolymers of low density (high comonomer content) while operating at economically preferable high polymerization reaction temperatures and high polymer production rates is highly desirable.